Numerous machines are known for grinding and polishing optical lenses and, in particular, lenses having toric surfaces. The term toric surface may be understood as meaning a non-rotationally symmetrical, aspheric surface, which has two different radii of curvature in two mutually perpendicular sections, where the other sections represent curves of a higher order.
The finish-grinding and polishing of blanks for toric lenses is usually carried out by means of tools which have a continuous or uninterrupted, toric surface complementary to the desired lens surface. It is important, especially in the context of correcting lenses, that the two circular-arc profiles of the toric lens are mutually perpendicular. During the course of the final grinding and polishing any relative rotation between the torus axes of tool and lens must be avoided. Put another way, the two special, mutually perpendicular planes of the grinding or polishing tool, which are moved over the surface of the lens at any given point, must always remain parallel to themselves, i.e., unchanged in their orientation. Accordingly, devices for finish-grinding and polishing commonly utilize a tool in varied orbital movements, but without any rotational movement about an axis perpendicular to the toric surface.
Conventional devices for finish grinding and polishing of toric lenses typically possess a guide device and an entraining device. The guide device is intended to prevent such rotational movements and the entraining device assures a change in position of the tool relative to the lens. The-entraining device basically consists of a ball joint positioned in the axial direction in which the attack force of the tool-acts upon the lens. The ball joint deflects movements, which would otherwise cause a relative change in position between lens and tool, into a direction perpendicular to its axis. The ball joint thus makes possible a cardanic compensation between lens and tool.
A number of guide devices are known which operate on this principle. Among the known guide devices are those in which either the lens or the tool is held fixed, while the other element, that is the tool or lens, is guided by forks, drive rods and/or joints, such as ball joints. In a similar design, either the lens or the tool is caused to perform a rotational movement while the other element, that is tool or lens, is guided by a device which possesses forks, drive rods and/or joints. A problem with both of these designs is that the fork tends to an eccentric setting which inhibits complete realization of the torus.
Another problem is conventional devices for grinding and polishing typically consist of many mechanical components. Toric surfaces of good quality can be obtained using these devices, provided they are correctly adjusted. However, during operation, the components of these complicated mechanical guide devices are exposed to the abrasive grinding and polishing agents. As a result, the various components wear quickly and play develops in the joints between the components. This play arises in every type of joint, but is especially problematic in bearing and fork joints. This wear phenomena results in a change of the characteristics of the surface of the lenses produced. In particular, the relative orientation the two circular profiles of the lens can change such that the profiles are no longer be orientated perpendicularly to each other. Moreover, because considerable masses needs to be moved in conventional devices, this wear phenomena limits the relative speed between tool and workpiece and thereby results in lowering the achievable volume-time ratio and hence increasing the machining time.
Another known device includes a seating chuck equipped with either a lens support or tool support. A connecting component consists of a journal and a bell-shaped flange for fitting of the chuck. A ball joint connects the support to the connecting component. A roller bellows, non-resilient in the circumferential direction, connects the flange to the support and seals the space (joint chamber) in which the ball joint is situated. A pressure fluid line connected to the joint chamber. An axially movable spindle sleeve is disposed on the grinding or polishing machine. The seating chuck is concentrically fixed to the inner end of the spindle sleeve.
An important advance in respect of the problems indicated has been provided by a chuck for optical lenses disclosed in German Patent Specification 22 52 503. In this chuck, the orientation of the torus axes is not achieved by forks and the two conventional ball pins. Rather the chuck includes a form-fitting lens support that is guided by a roller bellows which ms non-yielding in the circumferential direction. The roller bellows presses the lens support and the lens to be machined against the tool (or vice versa) by means of an air cushion. The chuck can be concentrically fixed to the inner end of an axially movable spindle sleeve on a device such as a Loh Toro-X 2000, grinding and polishing machine as provided by the Firms Loh Optikmaschinen KG of CH-4702 Oensingen and Wilhelm Loh Wetzlar Optikmaschinen GmbH & Co KG of DE-6330 Wetzlar. Hence, the lens support is axially displaceable. Radial guidance is provided by a pin having a spherical tip about which the lens support can execute an oscillating movement which is necessary for the formfitting, homokinetic adaptation between tool and lens. The bearing pressure that acts here is transmitted via the roller bellows to the lens support. The entire chuck thus forms a homokinetic coupling, which makes possible a transmission of torques between lens support and tool without play and at the same time hermetically seals the ball joint against the abrasive polishing and grinding agents.
A major disadvantage of this device is that the spindle sleeve must be axially adjusted by hand in order to ensure correct functioning of the seating chuck.
This is especially problematic because the differing lens thicknesses and tool heights usual in machining glass to prescriptions, require that the position of the spindle sleeve must be reset each time a new lens is machined or new tool is used.